1. Field of the Invention
The present invention relates to a method of reflowing a semiconductor device so as to flatten an insulating film between wiring layers in a multilayer wiring process. More particularly, the present invention relates to a method of reflowing a semiconductor device so as to effectively flatten an insulating film using a two layer borophospho silicate glass (BPSG) film, the two layer insulating film being successively deposited between wiring layers using a plasma in an in-situ state.
2. Background Information
Generally, a conventional reflow process comprises the following steps as shown in FIG. 1(A) to (F).
An insulating film 2a is formed on a substrate 1 and then an electrode 3 is formed on the insulating film 2a. Next, an insulating film 2b is formed on the insulating film 2a and the electrode 3.
A borophospho silicate glass film 4 is deposited over the insulating film 2b using a compounded gas formed from sillen (SiH.sub.4) gas, a diboron (B.sub.2 H.sub.6) gas, and a phosphine (pH.sub.3) gas in an atmospheric pressure chemical vapor deposition (APCVD) apparatus.
The borophospho silicate glass film 4 comprises a structure containing 3-4wt. % boron and 5-7wt. % phosphorus. The borophospho silicate glass film 4 is simply called a BPSG film hereinafter.
As a result, negative region 5 shown in FIG. 1(C) is formed. Although the BPSG film 4 is reflowed at 900.degree. C., the insulating film cannot be effectively flattened. If the boron and phosphoric concentration of BPSG film 4 is increased, the reflowing temperature of BPSG film 4 decreases.
However, if the boron concentration is increased, surface crystallization of BPSG film 4 results. If the phosphoric concentration is increased, hygroscopicity of BPSG film 4 is strengthened and an acid by-product will form which can corrode adjacent metal wiring layers.
In conventional semiconductor devices using BPSG insulating film, selection of boron and phosphoric concentrations are thus limited and reflow temperatures cannot be successively lowered below 900.degree. C. High reflow temperatures are a great disadvantage.
A conventional reflow process is shown in FIG. 1(A) to (F). After BPSG film 4 has been deposited, a phosphor silicate glass PSG film 6 containing 9 wt. % phosphorus is deposited to a thickness of 1500 to 2000 .ANG. over BPSG film 4, as shown in FIG. 1(D). Subsequently, the semiconductor device is reflowed at 900.degree. C. in a nitrogen gas or "pocl.sub.3 " environment and PSG film 6 is etched with a diluted fluorine acid (H.sub.2 O:HF=100:1).
Although negative region 5 disappears and the semiconductor device is made planar, the etching of PSG film 6 results in the high concentration of phosphorus in PSG film 6 diffusing into BPSG film 4 during the reflowing step. As explained above, the high phosphoric concentration results in the corrosion of on-chip metal wiring. Also, the temperature remains relatively high.
Also, as shown in FIG. 3, conventional processes are disadvantageous because a degree .THETA. of a flattened insulating film, corresponding to an angle of non-planarization, is a relative high value of 30.degree.-40.degree..